Emissive flat-panel display devices are widely used in conjunction with computing devices and in particular with portable devices. These displays are often used in public areas with significant ambient illumination and are viewed from a wide variety of angles.
Light emitting diodes (LED) incorporating thin films of organic or inorganic light-emitting materials have many advantages in a flat-panel display device and are useful in optical systems. U.S. Pat. No. 6,384,529 issued May 7, 2002 to Tang et al. shows an OLED color display that includes an array of organic LED light-emitting elements (pixels). Light is emitted from a pixel when a current is passed through an organic material, the frequency of the light is dependent on the nature of the organic material used. The organic materials are placed upon a substrate between electrodes, with an encapsulating cover layer or plate. In such a display, light can be emitted through the substrate (a bottom emitter) or through the encapsulating cover (a top emitter), or both. The emitted light is Lambertian, that is it is emitted equally in every direction. Because LED devices employ high-optical-index emissive materials, a large fraction (e.g. greater than 50%) of the emitted light is trapped in the device due to total internal reflection and thus reduces the device efficiency. Inorganic materials, for example, may include phosphorescent crystals or quantum dots. Other thin films of organic or inorganic materials may also be employed to control charge injection, transport, or blocking to the light-emitting-thin-film materials, and are known in the art.
LED devices can comprise a patterned light-emissive layer wherein different materials are employed in the pattern to emit different colors of light when current passes through the materials. However, patterning the materials, particularly for small-molecule organic LED materials is difficult for large-size substrates, thereby increasing manufacturing costs. Moreover, existing patterning methods, for example employing metal shadow masks, are expensive and can damage deposited organic materials. One approach to overcoming material deposition problems on large substrates is to employ a single emissive layer, for example, a white-light emitter, together with color filters for forming a full-color display, as is taught in U.S. Pat. No. 6,987,355 entitled, “Stacked OLED Display having Improved Efficiency” by Cok. It is also known to employ a white sub-pixel that does not include a color filter, for example, as taught in U.S. Pat. No. 6,919,681 entitled, “Color OLED Display with Improved Power Efficiency” by Cok et al. A design employing an unpatterned white emitter has been proposed together with a four-color pixel comprising red, green, blue color filters and sub-pixels and an unfiltered white sub-pixel to improve the efficiency of the device (see, e.g. U.S. Pat. No. 7,230,594). However, these disclosures do not address the problem of large amounts of trapped light.
Regardless of the light-emitting layer structure, area LED devices suffer from trapped light in the light-emitting layer, due to the relatively high index of the emission layer compared to air. Optical cavity structures are known to increase the light emitted from an OLED device structure. Such optical cavity structures are also known as microcavities or optical microcavities when formed in thin films. When formed in LED devices, different color light-emitting organic materials are pattern-wise deposited over a substrate between a reflective electrode and a semi-transparent electrode. Light emitters having different colors are thus formed within an optical cavity tuned to a desired peak wavelength of light, typically corresponding to the color of light emitted by the patterned organic materials. U.S. Pat. No. 6,680,570 describes an organic light-emitting device with improved color control employing spacer layers to form an optical cavity. FIG. 5 illustrates such a prior-art, active-matrix, bottom-emitting optical cavity device employing a substrate 10 with active-matrix thin-film electronic components 30, planarization structures 32 and 34, and a semi-transparent electrode 16. Patterned organic materials 14R, 14G, and 14B, providing red, green, and blue light emission, are deposited in a light-emitting layer 14. Optical spacers 26R, 26G, and 26B are employed to form optical cavities 60, 62, and 64 tuned to the desired peak wavelengths of red, green, and blue light, respectively to emit red light 80, green light 82, and blue light 84. A cover 20 can be employed to protect and encapsulate the device. While such designs are useful, they require a patterned organic material deposition technology (for example, vacuum deposition through metal shadow-masks) that is difficult to scale to large substrates.
To overcome the cost of patterned deposition, one can employ an unpatterned broadband (e.g. white) emitting layer for all sub-pixels, together with color filters for three color sub-pixels and an unfiltered, fourth white emitter, as noted above. However, the problem of trapped light remains. If a microcavity design employing tuned microcavities for each colored emitter is employed to extract trapped light, the fourth emitter will no longer emit white light, due to the frequency selection inherent in optical cavity structures. The loss of a white emitter reduces the efficiency of the unpatterned device. Furthermore, the construction of optical cavities tuned to the color of each emitter can be expensive, requiring multiple manufacturing steps.
U.S. Pat. No. 5,554,911 entitled, “Light-emitting elements” describes a multi-color light-emitting element having at least two optical cavity structures with respectively different optical lengths determining their emission wavelengths. Each optical cavity structure includes an organic material as a light-emitting region, which may be a single film of uniform thickness in the element. U.S. Pat. No. 6,861,800 entitled, “Tuned microcavity color OLED display” describes a microcavity OLED device having an array of pixels divided into at least two different color pixel sets, each color pixel set emitting a different predetermined color light over a common substrate, wherein each pixel in the array includes a metallic bottom-electrode layer disposed over the substrate and a metallic electrode layer spaced from the metallic bottom-electrode layer. The material for the semi-transparent metallic electrode layer includes Ag, Au, or alloys thereof. The thickness of the semi-transparent metallic electrode layer, the combined thickness of the organic layers, and the transparent conductive phase-layer; and also the placement of the light-emitting layer are selected so that each pixel in the display forms a tuned microcavity OLED device having emission output efficiency above that of a comparable OLED device without the microcavity. U.S. Pat. No. 5,949,187 describes an OLED with a first microcavity including a first transparent spacer and a first mirror stack positioned on the first spacer to reflect light back into the OLED and to define an optical length of the first microcavity. The optical length of the first microcavity is such that light emitted from the first microcavity has a first spectrum. A second microcavity includes a second transparent spacer positioned adjacent the first microcavity and a second mirror stack positioned on the second spacer reflects light toward the second OLED and defines an optical length of the second microcavity. The optical length of the second microcavity is such that light emitted from the second microcavity has a second spectrum. Additional microcavities can be placed in the structure to further enhance and alter the light spectrum. Such designs, however, do not address the need for reduced manufacturing costs and increased efficiency.
US 2006/0066228 A1 entitled, “Reducing or eliminating color change for microcavity OLED devices”, by Antoniadis discloses a microcavity OLED device that minimizes or eliminates color change at different viewing angles. The OLED device can be, for example, an OLED display or an OLED light source used for area illumination. This OLED device includes a multi-layer mirror on a substrate, and each of the layers is comprised of a non-absorbing material. The OLED device also includes a first electrode on the multi-layered first mirror, and the first electrode is substantially transparent. An emissive layer is on the first electrode. A second electrode is on the emissive layer, and the second electrode is substantially reflective and functions as a mirror. The multi-layer mirror and the second electrode form a microcavity. On a front surface of the substrate is a light modulation thin film. The light modulation thin film can be any one of: a cut-off color filter, a band-pass color filter, a brightness enhancing film, a microstructure that attenuates an emission spectrum at an angle at which there is a perceived color change, or a microstructure that redistributes wavelengths so the outputted emission spectrums have the same perceived color. Again, such designs may have increased manufacturing costs due to patterned deposition processes. Also, significant light may be absorbed by the color filters thereby, reducing efficiency.
U.S. Pat. No. 7,030,553 entitled, “OLED device having microcavity gamut sub-pixels and a within gamut sub-pixel” by Winters et al, discloses an example of a prior-art microcavity device. This disclosure describes an OLED device including an array of light-emitting pixels, each pixel including sub-pixels having organic layers including at least one emissive layer that produces light and includes spaced electrodes. There are at least three gamut sub-pixels that produce colors that define a color gamut and at least one additional sub-pixel that produces light within the color gamut produced by the gamut sub-pixels. At least one of the gamut sub-pixels includes a reflector and a semitransparent reflector, which function to form a microcavity. However, this design employs a patterned semi-transparent electrode that can be difficult to manufacture in a top-emitting format. U.S. Pat. No. 6,570,584 entitled, “Broad Color Gamut Display” by Cok et al describes a digital color image display device, including a plurality of pixels, each pixel having a plurality of sub-pixels with at least one of the sub-pixels producing a color other than red, green, or blue. However, there is no teaching of improving device efficiency. US 2006/0192220 describes a bottom emitter device having a patterned reflection film to form both light-emitting regions having a microcavity and light-emitting regions without a microcavity. This particular structure requires a patterned reflection film, thereby increasing costs. The structure is also not useful for unpatterned light-emitting layers.
There still remains a need, therefore, for an improved light-emitting structure that overcomes shortcomings in the prior art and that increases the light output and reduces manufacturing costs of an LED device, such as a display.